Recently, attention has been paid to a spread spectrum communication method which is advantageous in preventing interference or jamming, as one of communication methods applicable to a mobile radio communication system.
With the spread spectrum communication method, a code division multiple access (CDMA) system is realized. For example, in a transmission-side apparatus, digitized voice data and image data (base band signals) are subjected to modulation by a digital modulation method such as a PSK or FSK modulation method. Then, the modulated transmission data is spectrum-spread by using spread codes such as pseudo-noise codes and converted to a wide-band signal. The wide-band signal is then converted to a radio frequency signal and transmitted. On the other hand, in a reception-side apparatus, the received radio frequency signal is frequency-converted to an intermediate frequency signal or a base band frequency signal. Then, the frequency-converted signal is inversely spread by using the same codes as the spread codes used in the transmission-side apparatus. Thereafter, digital demodulation is performed by a digital demodulation method such as a PSK or FSK demodulation method. Thus, the reception data is reproduced. By varying the spread codes, a plurality of transmission/reception apparatuses can be used simultaneously with the same frequency.
In this type of CDMA system, a so-called "near-far problem" occurs due to the distance between a base station and a mobile station. Specifically, this problem occurs when a plurality of mobile stations simultaneously perform communication with the same frequency. In this case, the power of electric waves transmitted from the mobile station located near the base station is greater than the power of electric waves transmitted from the mobile station located far from the base station. As a result, the base station cannot receive the radio waves from the far mobile station, owing to the high-power radio waves from the near mobile station. Consequently, radio connection control between the base station and the far mobile station cannot be performed, or the quality in communication is degraded.
In the prior art, in order to solve this problem, the following technique has been proposed. That is, the intensity of a received electric field is measured in the mobile station or the base station. On the basis of the measured result, the transmission power of the mobile station is controlled by an open-loop control method or a closed-loop control method. Irrespective of the distance between the base station and the mobile stations the base station can receive at all times the electric waves from the mobile station at a constant reception power.
In the open-loop transmission power control methods the electric field intensity (received electric field intensity) of the constant-power radio frequency signal transmitted from the base station is measured in the mobile station. Based on the measured value, the mobile station determines the transmission power of itself. Specifically, when the received electric field intensity of the radio frequency signal coming from the base station is low, it is determined that the distance between the base station and the mobile station is large, and the transmission power is set at a high value. On the other hands when the received electric field intensity of the radio frequency signal coming from the base station is highs it is determined that the distance between the base station and the mobile station is small, and the transmission power is set at a low value.
On the other hands in the closed-loop transmission power control methods the electric field intensity (received electric field intensity) of a constant-power radio frequency signal transmitted from the mobile station is measured in the base station. Based on the measured values the base station generates command information to control the transmission power of the mobile station and sends the information to the associated mobile station via a forward link. The mobile station controls the transmission power of itself in accordance with the command information.
These control methods, however, have the following problems. In the mobile radio communication systems such as an automobile telephone system and a portable telephone system, different radio frequencies are generally used in a forward link from the base station to the mobile station and in a reverse link from the mobile station to the base station. Thus, in the open-loop transmission power control method, the frequency of the radio signal with which the mobile station measures the received electric field intensity differs from the frequency of the radio signal for actual transmission. Thus, the distance between the base station and the mobile station cannot exactly be measured. The transmission power cannot properly controlled owing to the influence of frequency selectivity fading, etc.
In the closed-loop transmission power control method, on the other hand, the influence due to frequency selectivity fading, etc. can be eliminated. Therefore, more proper control can be carried out, than in the open-loop control method. However, since the closed-loop transmission power control method is time-consuming, this method is generally performed only when the base station and mobile station are connected by a speech channel. Thus, the open-loop control must be relied upon before the radio link due to the speech channel is formed, i.e. while the radio connection control between the base station and mobile station is performed by using an access channel. As a result, the influence of frequency selectivity fading, etc. cannot be avoided. Furthermore, in this method, command information needs to be contained in a sync signal transmitted from the base station to the mobile station, the quantity of the information of the sync signal decreases that much.
As has been described above, in the conventional CDMA radio communication system, the influence of frequency selectivity fading, etc. cannot be avoided, or the transmission power cannot properly be controlled unless during the speech time period. Thus, the "near-far" problem is not solved, resulting in degradation in quality of speech or in unprecise radio connection control.
The object of the present invention, therefore, is to provide a mobile radio communication system capable of effectively solving the "near-far" problem without precisely controlling the transmission power of the mobile station, thereby enhancing the quality of speech and the reliability of radio connection control.